In one prior connection, known as a star connection, each of the devices at a working station is connected, by a cable associated with it, to the data processing unit with which the devices need to exchange data. The exchange of data between the unit and each device is the responsibility of pairs of transmitting and receiving circuits located at respective ends of the cable which connects the unit to the concerned device. Inputs and outputs to and from the unit are generally managed by running a program which is recorded in a memory of the unit. If the number of devices is large, then there is a large number of data transmitting and receiving circuits. This being the case, the amount of memory space required by the program is that much greater, to the detriment of the space which is left in the memory for performing routines which indicate the processing capacity of the unit.
In another prior connection, the various devices at a given working station are connected in series to the unit by a set of lines forming a single channel. Data between the unit and the device are exchanged along this connection, character by character, by transmitting the bits of each character in parallel. The advantage of this connection in comparison with a star connection is that it reduces the number of data transmitting and receiving circuits situated at the data processor end of the cable, whereby there is a reduction of the memory space for the program which manages the inputs and outputs of the processor.
In many applications, such as banking data-processing, slow devices are used at a peripheral working station. Slow devices are defined as devices which are only able to transmit and receive data at a speed lower than the speed with which the data processing unit is able to transmit and receive data. This being the case, it is not necessary for data which are exchanged during a dialogue sequence between the unit and a device to be transmitted at a speed higher than that at which the unit transmits and receives data. An interface particularly suitable in this instance is described in commonly assigned French patent application no. 7431212, filed on Sept. 16, 1974, which corresponds to U.S. application Ser. No. 622,466, filed Oct. 15, 1975. With a reduced number of transmission lines, and data transmitting and receiving circuits, and thus with a minimum amount of memory space required for the input/output management program, the required exchange of data occurs between slow devices and a processing unit, which may be a simple micro-processor. Such an interface enables a peripheral device, selected by the unit when a dialoguing sequence is initiated, to be operated as a data transmitter and a data receiver. However, with the prior art interface, if the data processing unit is required to transmit data received from a first device to another device, the unit must initiate another dialoguing sequence in which the other device is selected as a data receiver. For two devices to be able to exchange data through the circuit, it is therefore necessary for the unit to initiate two successive dialoguing sequences, whereby the respective initiating instructions for the two sequences must be stored in the memory of the unit.
One of the objects of the present invention is to enable data to be exchanged through a data processing unit between two devices at the same station using a minimum of means contained in the unit to which the station is connected by two transmission lines.
Another object of the invention is to enable data to be reliably exchanged between two devices at the same station which is connected to a data processing unit by two transmission lines.